1. Field of the Invention
This invention relates generally to control of a gearshift produced by an automatic transmission, particularly to the early stages of a synchronous gearshift.
2. Description of the Prior Art
A hydraulically-actuated transmission clutch features non-linear dynamic response in the early stages of its application. There is no on-board feedback signal that indicates the clutch state (e.g., piston position), until the clutch acquires torque transmitting capacity. The control algorithm commands the pressure at a control valve that is remotely located with respect to the clutch cylinder and piston. Open-loop boost and stroke pressure commands to the valve are used to control the clutch in these early stages. The duration of boost and the level of stroke pressure can be adapted in subsequent shifts, but imperfect shifts usually occur first, in order to invoke adaptation.
Knowing input shaft torque or output shaft torque is beneficial for improving the quality of gear shift control of an automatic transmission for a vehicle. Measurement of input shaft torque or output shaft torque allows feedback control that is more robust to unknown disturbances to be utilized. However, the conventional approach of error-based closed-loop (CL) control has limited authority during fast transients in the torque-transfer phase of a gearshift due to the clutch actuators' considerable nonlinearity, delay and lag.
A method for determining clutch torques during a gearshift using references to input and output shaft torque measurements or estimates has provided multiple opportunities for improving shift control exist. While the off-going clutch torque signal and on-coming clutch torque signal provide valuable feedback, essential for changing the whole paradigm of synchronous torque transfer control, that feedback is still not available during the initial phases of clutch actuation.
The clutch has torque carrying capacity only after certain nonlinear dynamic transients in the clutch actuator take place and, unfortunately, there is no feedback information during that transient response. To be able to command the clutch actuator during those initial phases in a robust fashion, knowledge of the internal state of the actuator would be essential.